Fire-detecting system



Oct. 2, 1945. F. c. EVANS ETIALV FIRE DETECTING YSTEM Filed March 6, sSheets-Sheet i FIG, 1

FIG. ;a

INVENTORS 9 .F. C. EVANS K.O. DONELIAN ATTORNEY FIG. 5

0t.2,19451 F. c. EVANS ET AL- 2,385,976

FIRE DETECTING SYSTEM Fi led March 6, 1940 3 Sheets-Sheet 2 FIG. 6

*WIIIIII I u J: INVENTORS I o F.C. EVANS i i BY K.O.DONELIANI d c b aPLATE CURRENT vs GRID VOLTAGE. ATTORNEY Oct; 2, 1.945.

F. c. EVANS ET.AL

FIRE DETECTING SYSTEM 3 Sheets-Sheet 3 Filed March 6, 1940 FIG. 7

FIG. ll

INVENTORS F c EVANS BY K10. DONELIAN ATTORNEY Patented a. 2, 1945FIRE-DETECTING SYSTEM Francis 0. Evans, Dongan Hills, and Khatchik 0.

Donellan, New York, N. Y., assignors to American District TelegraphCompany, Jersey City,

N. J., a corporation of New Jersey Application March 6,1940, Serial No.322,590

' 3 Claims. (Cl. 177-355) This invention relates generally to firedetection systems and more particularly to systems adapted to detect thepresence of fire in high velocity streams ofv air such as in airconditioning systems or around the motor of an airplane where enormousquantities of air circulate at high velocity.

The fire detection problems in air conditioning systems and airplanesare closely related because a large volume of air is always circulatedat high velocity through or around the most dangerous sources of fire..In an air conditioning system fires are very likely to occur in or inclose proximity to oil filters while in airplanes fires usually occuraround the motors and to the rear thereof. In both cases there are highvelocity air currents present which prevent any appreciableinstantaneous rise in temperature of the surrounding air itself. Thus itis very difficult to detect the presence of fire by means of temperaturesensitive devices such as aero tubing or thermostatic devices. Inairplanes it is necessary to provide instantaneous fire detecting meanssince the large supply of air causes excessively high' temperatureflames at certain points in the rear of the motor in a very short timeinterval. In fact the fire must be detected before the flame melts orreduces the strength of the surrounding structure of the airplane or thefire detecting apparatus itself.

' In air conditioning systems fires sometimes occur in the air filtersand it is necessary that the flame itself be detected, as the fire isusually confined which does not depend upon a predetermined rate of riseof temperature, whereby instantaneous fire detection is providedregardless of temperature change.

It has been found that the best method of detecting fire in accordancewith the above objects is to use the principle involving ionization ofair by a flame. Thus it-is possible to provide an electrode under thecowling of an airplane and spaced from the airplane structure so thatwhen a flame passes around the electrode, a circuit between theelectrode and the airplane frame is produced thus initiating an alarm.In this type of system the smallest fiame ionizes the gap between theelectrode and the airplane structure to causean instantaneous alarmthereby making it possible to prevent a high temperature from buildingup and the consequent destruction of the airplane structure. Instead ofa single electrode, a pair of electrodes may be mounted within thecowling to accomplish the same result.

In air conditioning systems it has been found that fires 'are likely tooccur in the air filters which use oil as a filtering medium, orwherever a filter can gather combustible material upon itself. Hereagain a high velocity ai-r current presents a difiicult fire detectionproblem in that temperatures do not build up at or near the source ofthe fire and consequently the use of rate of rise to one section of thefilter and therefore the enormous quantity of air circulated through thefilter prevents an appreciable change of temperature at other parts ofthe system where extinguishing apparatus such as sprinkler heads arelocated.

Accordingly the principal object of this invention is to provide a firedetecting system which operates instantaneously upon the occurrence of asmall flame.

A further object of this invention is to. provide a fire detectingapparatus suitable for use in airplanes or air conditioning apparatuswhich does not cause false alarms due to ambient heat such as that fromhot exhaust pipes, heating units,

or other local sources of heat.

' A further object of this invention is to provide a fire detectingsystem wherein the detector is perature for any appreciable length oftime and detection. devices or thermostatic devices is not feasible.However, by using either the wire screens of the filter itself, or anumber of external, spaced wire screens to create an air gap which maybe ionized by a flame, it is possible to instantaneously detect theexistence of a fire condition in spite of the high velocity air streamsand the lack of change in temperature.

galvanometer relay as an alarm relay and a troublerelay.

It is r .so a further object of this in\ ention to provide-circuitsincluding fiame detecting electrodes which are adapted for the use ofvacuum tubes and associated relaysfor not only detect-' lation totheplane structure;

Fig. 2 is a sectional view taken on line 22 Fig. 1;

Fig. 3 is a fragmentary view of a port on of Fig. 1 showing a furthermodification of this invention;

Fig. 4 is a perspective view of an air duct in section having protectivescreens supported ad- Jacent to the filters;

Fig. 5 is a perspective of a modification of the invention shown in Fig.4;

Fig. 6 illustrates a simple circuit involving the use of fire detectingelectrodes; I

Fig. 7 illustrates a Wheatstone bridge application of the invention;

Fig. 8 illustrates a direct current amplifier and an alarm circuit;

Fig. 9 illustrates an alternating current amplifier and alarm circuit;

' Fig. loillustrates the plate-grid characteristic of the circuits shownin Figs. 8 and 9; and

Fig. 11 illustrates the application of flame detecting electrodes in aconventional alarm circuit.

This invention has two important applications in which it isparticularly adaptable for the use of detecting flames, the first beingin connection with fire detection in airplanes while the second is inconnection with fire detection within air conditioning systems,particularly around air filters which contain oil as a filtering mediumor gather combustible materials.

Figs. 1, 2 and 3 01 the drawings illustrate the application of thisinvention to an airplane which comprises a body structure I, a pumpcompartment 2 and a motor compartment 3, the motor compartment beingsurrounded by cowling 4;

Since the air blast from the propeller carries the name to the rear ofthe motor the best location for fire detecting apparatus is in the rearof the motor and in such a position that the detectingapparatus-intercepts the path of the air flow and the path taken by theflames. Therefore a pair I of name detecting electrodes 6 may besupported on insulators I on the forward wall of the pump compartment 2and between the wall and the rear portion of the cowling 4. Theelectrodes 6 are positioned as shown in Fig. 1 of the drawings toencircle the outerportion of the pump compartment wall. Thus as flamesfollow the path of the air currents they ionize the air gap between theelectrodes and initiate an alarm in an electrical circuit 9 connectedthereto which will be described in detail later on in the description.

Another electrode -6, shown in Fig.6, may be supported on insulators IIon the fire wall at the rear of the pump compartment 2, this also pairof electrodes such as B or a single electrode and the body structure Iof the airplane can be substituted for, one another as flame detectorsdepending upon the requirements in the particular location selected fordetecting a fire condition.

Because of the inherent characteristics of the structure explained abovea fire is detected instantaneously since the flames themselves arecarried by high velocity air currents to the electrodes and the flamesionize the air gap to cause an alarm. It is obviously not necessary thata predetermined temperature occur for a certain length of time since theapparatus does not depend upon temperature change i'or its operation.

The high velocity air currents aid fiame detection apparatus since theyconvey the flames whereas,

if temperature responsive devices were utilized for detecting the fire,the high velocity air currents would prevent a rise of temperature to adegree suificient for indicating a fire condition. Furthermore,exceedingly high\ temperatures occur around an airplane motor and idlingperiods of the motor would cause false alarms in temperature responsivedevices.

Figs. 4 and 5 illustrate the application of the invention to an airconditioning system where it is desired to detect fires either withinthe filters or in the ducts. Fig. 4 of the drawings shows an air filterI5 which maybe of the type which utilizes oil as a filtering medium andwhich is particularly subject to the occurrence of fires therein.Obviously, other types otfilters may also be protected by this systemand therefore the invention is not limited to oil filter protection. The

fire detecting electrodes consist of wire screens I 6a which are placedon either side of the air filter ii to intercept the flow of air alongthe duct II, the duct or filter being used as one electrode wherebyoccurrence of a flame between the screens 6a and the 'duct or filterwillionize the air gap and cause an alarm in the associated electrical Ialarm circuit.

An alternative arrangementin-an air conditioning system is shown in Fig.5 of the drawings and includes an air filter l5 and two pairs oi screensto which are placed On either side of the filter l5, each pair ofscreens constituting a rounded metallic structure may serve as the otherelectrode to provide an ionizable air gap. Here again the flamedetecting electrodes 8a and 61) present the same advantages as inconnection with the airplane application shown in Figs. 1, 2 and 3,since enormous quantities of air fiow through the filters 15 at highvelocity and prevent high temperatures from occurring therein eventhough a fire may exist in the filters themselves or in the duct. Thisinvention is also particularly adapted to air conditioning systems sinceit is customary to place sprinkler heads in the filters and in theirimmediate vicinity for the purpose of ,deluging the'duct and the filterswith fire extinguishing medium.

It frequently occurs that the fire is confined to the filters or onesection thereof and the man velocity air currents prevent a rise oftemperature v reversed when it is desired to use a grounded cirsumcientto operate the sprinkler heads within the filters. This condition isprevented by providing control of the sprinkler system by the flamedetector, thus insuring that the sprinklerwithin the filter is operated.

Fig. 6 shows the simplest form of alarm circuit such as shown at 3 inFigs. 1 and 3 i'ordetecting flame by means of spaced electrodes. Thecircuit includes a source of potential 23 connected in series withcurrent limiting resistors 2|, galvanometer relay 22, and the spacedelectrodes 3 such as those shown in Figs. 1 to 5. Alarm bell 23 isconnected to the alarm contact 24 of relay 22 and trouble bell 21 isconnected to the trouble contract 23 of relay 22. Battery 23 suppliespotential for operating the alarm and trouble circuits. Resistor 23bridges electrodes 3 for the purpose of completing the supervisorycircuits whereby a supervisory current flows in the circuit. Theresistor23 may be of theorder of ten megohms or of sufllcient value toprovide apath of higher resistance than the path between electrodes 3 whenionized, so that when this latter path is ionized current can flowreadily thereacross.

When a flame passes between the electrodes 8 the air becomes ionized andconductive to suchan extent that sufllcient current flows through relay22 to move its armature from the normal neutralposition to contact 24thereby completing the alarm circuit through battery 23 and operatingalarm 26. The supervisory resistor 29 completes the series circuitacross electrodes 6 and allows a small supervisory current to flowtherein. Ifa break occurs in the circuit, the supervisory currentflowing through resistor 23 is reduced to zero and relay 22 is actuatedto close contact 25 and energize trouble signal 21 through battery 23.

The preferred circuit for use at 3 in Figs. 1, 3, 4 and 5 for providingflame detection is shown in Fig. 7 of the drawings and includes aWheatstone bridge comprising a galvanometer relay 22 connected betweenthe Junction of resistance arms 33 and II and the junction of resistancearm 33 and the remaining arm including the flame detecting electrodes 3.Battery 43 is connected across the other junction points in the bridge.The potentiometer is provided for normally balancing the Wheatstonebridge so that as a flame breaks down the air gap between the decuit.Thus in the case of a grounded circuit the switch 32 will establish aconnection through conductor 34, resistance 33, the lower electrode 3and resistor 23. Switch 32 establishes a'connection from the upperelectrode 8 which isnow the frame structure of the air plane or air ductto the grounded side of the battery. Such changes in the connection asabove described necessitate re-' versing the connections of galvanometerrelay 22 and therefore switches 42 and 42' are provided for reversingthe connection of relay 22 whereby it is always actuated in the samedirection regardless of whether or not a grounded circuit is used.Switches 42 and 42 are shown in the no-ground position and in the caseof a grounded circuit they would be operated to the other contacts in anobvious manner.

Galvanometer relay 22 controls the alarm relay which is connectedthrough alarm contact 24 of galvanometer relay 22 across the battery 31.

Relay 22 also actuates trouble relay 36 which is connected acrossbattery 31 in series with the current limiting resistor 38, relay 36being connected totrouble contact 25 of relay 22 in such a manner thatit may be shunted thereby. The alarm bell 23 is connected to the contact38 and armature of relay 33 and across battery 31 in series with onecontact of the silencing switch 40. Trouble buzzer 43 is also connectedin series with the same contact of switch and in series with thearmature and contact 4| of relay 36, the entire series circuit. beingconnected across battery 31. The trouble lamp 44 is connected acrossbattery 33 in series with the left-hand contact of. switch 40 wherebythis switch may control either alarm bell 23, trouble buzzer 43, ortrouble lamp 44.

In normal operation the bridge circuit is balancedwhereby the relay 22assumes a neutral tecting electrodes 3 the bridge becomes unbalanced andcloses the alarm contact 24 of relay 22.

The supervising resistor 23 is connected across the flame detectingelectrodes, and has a very high resistance of, for example, ten megohms.Thu a small current flowing through the supervising resistor 29 permitsthe bridge circuit to be balanced by closing the circuit across theflame detecting electrodes. If a trouble condition such as an opencircuit occurs in the electrode arm of the bridge,

it will become unbalanced and relay 22' will close the trouble alarmcontact 23.

-As mentioned before, it is possible to use two detecting electrodes orone detecting electrode in combination with the metallic structure orground of, an'airplane and an air duct-in an air conditioning system.Therefore in order to permit the use of a ground the bridge circuit isprovided with switches 32 and 32' together with the associatedconductors 34 and 34' for reversing the connection of the lowerelectrode 3 and resistor 33 to the remainder of the bridge circuit.Switches 32 and 32' are shown in their normal position.

where no ground is used and the switches will be armature to contact 24thereby closing a circuit for relay 35 across battery 31. Relay 35actuates its armature and makes contact 38 to close a circuit throughbell 26 and switch 40 whereby battery 31 energizes the bell to initiatean alarm. The silencing switch 40 is then actuated to silence the alarm.

When a trouble condition occurs, the bridge circuit becomes unbalancedin the other direction and relay 22 closes contact 25 which creates ashunt circuit around the normally energized trouble relay 36, therebyclosing the contact 4| to energize the trouble buzzer 43 through thesilencing switch40. Thus the switch 40 can again be opened to silencethe trouble buzzer and energize a pilot light 44. After an alarm Ortroufor the circuit to operate at a critical tempera- V ture or as aresult of a predetermined rate of rise in temperature. In addition thecircuit provides trouble supervision for informing the operator of anytrouble which might prevent the reception of an alarm.

Figs; 8 and 9 illustrate further modifications of this inventionincluding the flame detecting electrodes as used in connection withvacuum contact ll, while relay 35 operates the alarm bell 23 throughcontacts 38 and N.

Fig. 8 also illustrates the use of a flre detecting device 55 of theopen circuit type such as a thermostat or other heat responsive switchin combination with the fire detecting electrodes 8 and the amplifier30. The device 55 includes the contacts 53 and resistance 51 the purposeof which including a manually operated fire alarm station 33, a fixedtemperature fire detecting thermostat St, a water flow detector 32 and apneumatic rate of rise fire detecting unit 63. This circuit includesconductors 65 and 66 which are terminated at the terminals 31 and 68. Asupervisory resistor 29 is connected across terminals 69 and I0 and theother terminals 51 and 68 are connected to a circuit 9 such as shown inFi s. 1 to 9.

Thusii any of the fire detecting elements 6, Gil,

3!, 32 or 33 are actuated they will cause an alarm.

' If a break or a ground occurs on any oi. the

is to limit the grid bias and prevent" excessive decrease in platecurrent whereby only the alarm relay 35 is operated. If contacts 56 wereto place full grid battery potential on the grid of tube 30,; troublerelay 36 would be operated instead of the alarm relay 35.

Fig. 9 illustrates the application of a flame detecting electrode to analternating current vacuum tube circuit which includes a source ofalternating current 80 having the vacuum tube 30 connected in series.The fire detecting elecv trodes B are connected to the grid circuit oftube 50 and the trouble relay 33 and the alarm relay 3! are connected inthe output of tube 53. The relay contacts 33, 4|, and 54 and the alarmsignals 2'5 and 21 are connected as described above in connection withFig. 8.

The operation of the circuits illustrated in Figs. 8 and 9 may be betterunderstood by referring to Fig. 10 which shows the plate current-gridvoltage characteristic of the vacuum tubes used in the circuits. Maximumplate current occurs at zero bas on the grid of the tubes 30, this beingthe normal condition when there are no breaks within the circuit and noflame between the electrodes 6. Thus relays 35 and 33 are normallyenergized. When a flame occurs between the electrodes 6, a negative biasappears on the grid sufllcient to reduce the plate current apredetermined amount to a value where the alarm relay 33 drops ofi andcloses its contact to initiate conductors the supervisory current whichnormally flows through the resistor 29 will be interrupted and thetrouble alarm will be set oil as described in connection with theremainder of the circuit.

. From the abovedescription it is evident that this invention provides afire detecting system which does not depend upon temperature change forits operation and is therefore particularly adapted to uses such as inairplanes or air conditioning systems where high velocity air currentsprevent substantial temperature changes.

It is further evident from the above description that even though thefire detecting electrodes inherently provide an open circuit device,supervisory equipment is provided and constant trouble supervision ispossible.

It is also evident from the above description of this invention that itprovides fire detecting apparatus which is not disabled by high velocityair streams and is not subject to false alarm conditions which may occurin such installations as those in airplanes or air conditioning systems.In other words, the apparatus fully accomplishes the results set forthin the objects or the invention.

an alarm at bell Z3. Relay 36 is designed to drop oil on less currentthan alarm relay 35 and does not drop on an alarm. Referring to Fig. 10the grid voltage would have a value such as 0 between pointsb and ddepending upon the resistance of the flame itself. At point (I the platecurrent will be sufilciently reduced to drop the trouble relay 33 alsobut by the proper selection of circuit resistances this point dis notreached in case of a flame passing between the electrodes. A ground onthe upper electrode 8, failure of tube Ill, failure of the currentsource, a short circuit between electrodes or a break in the platecircuit will put a greater bias on the grid of the vacuum tube or willdecrease the current in the plate circuit a greater amount so that boththe alarm relay 35 and the trouble relay 33 will drop off. The troublealarm 21 will be energized by contact ll, but the contact 54 will breakthe alarm circuit and therefore prevent a false alarm on bell 23.. Thusit is evident that the vacuum tube circuit as well as the electrodecircuit is supervised for trouble conditions at all times. It is obviousthat the heat responsive device 53 may 1 actuate the alarm in thesamernanner as electrodes 8.

Fig. 11 of the drawings illustrates the application of flame detectingelectrodes 3 to a circuit It is not intended that the fire detectingelectrodes be located only in the positions indicated in Figs. 1 to 5 ofthe drawings as they are equally adapted to be situated in any desirablelocation either in an airplane or in an air conditioning system. Anydangerous structure may be protected by the flame detecting electrodes.It is also not intended that the use of flame detecting electrodes shallbe limited to the specific circuits shown in Figs. 6 to 9 and 11, as itwill be obvious to those skilled in the art that a number of diiIerenttypes oi bridge circuits or vacuum tube circuits are equally adaptableto the use of electrodes. Various modifications and improvements armsthereof, a pair of flame detecting electrodes in the fourth arm thereofdefining a gap the air at which is normally non-conductive, asupervising resistor connected across said electrodes whereby the bridgeis normally balanced, a galvanometer relay connected across saidbridge'in such fashion that the occurrence of flame between saidelectrodes creates a conductive path substantially solely by ionizationof the air at said gap and unbalances the bridge to deflect the relay inone direction while a break anywhere in the circuit deflects the relayin the opposite direction.

2. In a signaling system, a source of potential, a balanced bridgecircuit connected across said source and comprising balancing resistorsin three arms thereof, a pair of flame detecting electrodes in thefourth arm thereof defining a gap the air at which i normallynon-conductive, a supervising resistor connected across said electrodeswhereby the bridge is normally balanced, a g-alvanometer relay connectedacross said bridge and having an alarm contact and a trouble contact, analarm relay connected to the alarm contact and a trouble relay connectedto the trouble contact whereby the occurrence of flame between saidelectrodes creates a conductive path substantially solely by ionizationof the air at said gap and unbalances the bridge to deflect the relay inone direction and energize the alarmrelay while a break anywhere in thecircuit deflects

